doc-rst: fix parsing comments and '{' on a separate line

The dmx.h header has two things that causes the parser to
break while handling enums:
 per-header enums and the '{' starts on a new line

Both makes the parser to get lexical marks to be detected
as if they were symbols.

Fix it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>